1. Field of the Invention
The present invention relates to switched mode power supplies, and, in particular, to a control circuit for controlling a switch which temporarily short-circuits the secondary diode of a switched mode power supply and reduces the power loss in this way. The invention especially relates to a control circuit in a primary-controlled switched mode power supply, which comprises a primary-side switch and a transformer with a primary winding, a secondary winding and a secondary-side auxiliary winding. A voltage tapping point is provided between the secondary winding and the auxiliary winding, the control circuit being adapted to be connected to a connection of the auxiliary winding and to said voltage tapping point. The control circuit is adapted to be connected to an output terminal of the switched mode power supply and the output voltage is adapted to be tapped parallel to the secondary winding. The invention additionally relates to a switched mode power supply for generating an output voltage, said switched mode power supply making use of the control circuit.
2. Description of the Related Art
Switched mode power supplies are clocked power supply units which chop a rectified, filtered mains voltage. Such switched mode power supplies are nowadays preferred to conventional power packs with power transformers for many cases of use, since, from a certain performance category onwards, they have a better efficiency and require in particular less space.
The use of electronic switches causes, essentially, only switching losses, the voltage transformed being, instead of the mains voltage, a high-frequency a.c. voltage. Instead of the normal mains frequency of 50 Hz or 60 Hz, the high-frequency a.c. voltage can e.g. be in the range of from 20 kHz to 200 kHz.
Since the necessary numbers of windings of the transformer decrease inversely proportional to the frequency, the ohmic losses can be markedly reduced in this way and the transformer required becomes much smaller. Control is effected either by varying the pulse duty ratio at a constant frequency or by varying the frequency at a fixed or variable pulse duty ratio.
The output voltage can be determined by the clock ratio with which the electronic switch is closed. The voltage chopped by the electronic switch can be transformed into any other voltage and rectified. When, for controlling a desired d.c. output voltage of a switched mode power supply, the pulse duty ratio of a switch is varied on the primary side, the switched mode power supplies in question are referred to as primary-clocked switched mode power supplies. Primary-clocked switched mode power supplies are normally preferred because of their higher efficiency.
In order to optimize the efficiency still further, especially primary-clocked switched mode power supplies are known in the case of which the frequency generated by the switch, e.g. a bipolar transistor, on the primary side of the high-frequency transformer is controlled in dependence upon the load applied to the secondary side of the power pack, so as to avoid saturation of the high-frequency transformer. The feedback required for this kind of control is realized e.g. in that a voltage tapped at an auxiliary winding is used as a reference value.
Generating a reference value by making use of an auxiliary winding is shown e.g. in German patent application DE 100 18 229 A1. The method described in DE 100 18 229 A1 and used for controlling the output current and/or the output voltage of a primary-controlled switched mode power supply comprises the use of a reference value so to as to make the current and voltage control independent of the input voltage, said reference value being formed within the circuit and used for influencing the voltage controller.
Switched mode power supplies of this kind often operate a secondary circuit comprising a plurality of diodes, capacitors and other electronic components. For converting the a.c. voltage generated by the switched mode power supply into a d.c. voltage, diodes are used. Since these diodes cause a non-negligible voltage drop during the current flow, an undesirable power loss will be caused. In order to reduce this power loss, a switch, e.g. a field effect transistor, is switched on parallel to the diode, if necessary. This has the effect that the voltage drop decreases and that the power loss will be reduced.
Such a circuit is referred to as synchronous rectifier. Such synchronous rectifiers require in some cases a considerable investment in circuit technology for controlling the switch; it is desirable to reduce this investment in circuit technology so as to be able to offer the switched mode power supply in a more compact structural design which can be produced at a more reasonable price.
Moreover, from WO99/5777 a continuous mode flyback converter is known, wherein an FET transistor is provided for reducing the voltage drop at the secondary side of the transformer. The use of the FET transistor makes the converter more suitable for low voltage applications since smaller power losses are imposed in the secondary side of the converter than in a conventional converter. The converter also has a DC-blocking capacitor for further reducing the power losses at the secondary side. The secondary side further comprises a PNP transistor, the emitter of which is connected to the gate of the FET transistor, the collector of which is connected to the source of the FET transistor, and the base of which is connected to the first terminal of the auxiliary winding.
This circuit, however, suffers from the problem that by providing said PNP transistor the circuitry is still too complicated and expensive, and that the reliability may still further be improved.